The present invention relates to a two-component diazotype material comprising a support and one or several light-sensitive layers, each of which contains at least one light-sensitive diazonium salt, a coupler component, and an acid stabilizer, wherein at least one of the layers contains a compound which absorbs light in the ultraviolet spectral region.
Two-component diazotype materials are known in the art. They generally comprise a coating containing at least one light-sensitive diazonium salt and at least one coupler component which couples with the diazonium salt in an alkaline medium to form an azo dye. The light-sensitive coating of diazotype materials may comprise one or several layers, and each of these layers contains at least one diazonium salt and at least one coupler component. In addition, the layers usually contain further additives, for example, acid stabilizers to prevent premature dye coupling, thiourea to improve the image background, and sterically hindered phenols to increase the fastness to light of the azo dyes, and further additives, which are conventionally used in diazotype printing.
Diazotype materials have a very steep gradation and they consequently yield copies of very high contrast. They are therefore particularly suitable for improving reproductions such as line drawings, book copies, and other large-surface black and white copies, e.g., halftone dot patterns, since upon imagewise exposure, the diazonium salt is generally completely destroyed in the areas which have been struck by actinic radiation. Due to this characteristic, diazotype materials are normally not suited for the reproduction of continuous tone originals, since they do not yield a true-tone reproduction of details in highlight image areas.
In order to obtain improved reproductions of continuous tone originals by means of such diazotype materials, it has already been proposed to use several diazonium salts of different light-sensitivities (British Pat. No. 726,755). The diazonium salt having the higher sensitivity to light is more rapidly decomposed so that contrast in the high-density image areas is increased. The diazonium compound which has the lower sensitivity to light remains undecomposed for a longer period of time and, as a result, contrast in the lower-density image areas is reduced. This method has, however, the disadvantage that the pairs of diazonium salts of different light-sensitivities form dyes of different color shades and will, moreover, couple at different speeds. Thus an image is produced in which the color shade generated in the high-density areas differs from that generated in the lower-density areas ("two-tone effect"). It is also a disadvantage that copying speed is adversely affected by the presence of the diazonium salt which is less sensitive to light.
It is also known to add a yellow dye to the light-sensitive layer, to render the sensitometric characteristics of the diazotype material prepared therefrom generally softer, i.e., to flatten gradation. This method has the disadvantage that a considerable quantity of yellow dye is necessary for contrast softening. The quantity of dye which is required reduces copying speed and leads, moreover, to a noticeable reduction in the overall contrast of the copy.
German Pats. No. 1,204,069 (corresponding to British Pat. No. 871,216) and No. 1,263,505 (corresponding to U.S. Pat. No. 3,365,296) disclose materials which make it possible to obtain a nearly correct rendering of tones in the reproduction of continuous tone originals. The diazotype material according to German Pat. No. 1,204,069 is constructed of several layers, one of which absorbs ultraviolet light. The diazotype material described in German Pat. No. 1,263,505 comprises a single layer which contains the diazonium salt, the coupler component and, in addition, an ultraviolet absorbing compound which effectively absorbs radiation in the spectral region from 300 to 460 nm and is changed by actinic radiation in such a way that it is no longer capable of absorbing radiation in this spectral region. These diazotype materials have the disadvantage that the compounds which absorb ultraviolet light produce an undesired staining in the highlight image areas, because they are either not decomposed or only incompletely decomposed by the action of light and are thus still present in the final copy. In addition, the ultraviolet absorbing compound which is still present in the developed copy renders the image extremely impermeable to ultraviolet radiation, resulting in undesirably long exposure times when the copy is used as an intermediate original.
German Pat. No. 2,035,392 (corresponding to U.S. Pat. No. 3,679,415) discloses a diazotype material for the reproduction of continuous tone originals which contains, as the ultraviolet absorbing compound, a specific pyrylium or thiapyrylium salt, which is completely bleachable in an alkaline medium. The disclosed diazotype material has an extended exposure latitude and shows hardly any staining in the highlight image areas. The material comprises several superimposed, light-sensitive layers, each of which contains at least one diazonium salt and at least one coupler component. At least one of these layers contains the alkaline bleachable pyrylium or thiapyrylium salt as the compound which absorbs ultraviolet light. The pyrylium and thiapyrylium salts, respectively, are, for example, obtained from acetophenone and benzaldehyde in 3 or 4 reaction steps which involve a very expensive technology and result in yields of only about 20 to 30%, based on the starting materials employed. (See K. Dimroth, Angewandte Chemie ("Applied Chemistry"), 72d year, 1960, No. 10, pages 331 to 342). Since it is relatively difficult to obtain these pyrylium and thiapyrylium salts, the commercial application of a corresponding diazotype material is limited.